NOTE: These are the main Borderlands 2 cheats that are usually applied by pressing down the right key combination. Please find the instructions on how to use these cheats below.

Infinite Eridium
Enter the 'Three Horns Valley' then head northwest to the Bloodshot hideout and kill the 'Bad Maw' boss. As soon as you take out Bad Maw, collect the Eridium, which he leaves behind. Once you enter the Bloodshot Stronghold head to the main menu and select 'Save and Quit'. Continue the game, go back to Three Horns Valley, deal with Bad Maw once more and grab additional Eridium. You can do this as often as you wish.

Hints

NOTE: As their name suggests hints are not exactly Borderlands 2 cheats, and are usually just simple (but very useful) tips on how you can unlock secret level, features, find hidden areas and such. Hopefully these will make playing Borderlands 2 even more entertaining.

Achievements
Complete the following achievements to unlock Xbox Live Gamerscore points.

First One's Free (10 points): Completed the mission "My First Gun".
Dragon Slayer (20 points): Completed the mission "Best Minion Ever".
A Road Less Traveled (20 points): Completed the mission "The Road To Sanctuary".
New In Town (20 points): Completed the mission "Plan B".
An Old Flame (20 points): Completed the mission "Hunting The Firehawk".
No Man Left Behind (20 points): Completed the mission "A Dam Fine Rescue".
Wilhelm Screamed (20 points): Completed the mission "A Train To Catch".
Sky's The Limit (20 points): Completed the mission "Rising Action".
Can See My House From Here (20 points): Completed the mission "Bright Lights, Flying City".
Farewell, Old Girl (20 points): Completed the mission "Wildlife Preservation".
Got The Band Back Together (20 points): Completed the mission "The Once and Future Slab".
Identity Theft (20 points): Completed the mission "The Man Who Would Be Jack".
An Angel's Wish (25 points): Completed the mission "Where Angels Fear To Tread".
Bombs Away (20 points): Completed the mission "Toil And Trouble".
Knowing Is Half The Battle (20 points): Completed the mission "Data Mining".
Cool Story, Bro (30 points): Defeated Jack.
What does it mean? (15 points): I can't even capture it on my camera.
Well That Was Easy (10 points): Completed the mission "Shoot This Guy in the Face".
Thresher Thrashed (30 points): Defeated Terramorphous the Invincible.
Feels Like The First Time (10 points): Opened the chest at the bus stop in Fyrestone.True Vault Hunter Mode
Complete Normal difficulty story mode.

DLC: Captain Scarlett and her Pirate's Booty
Complete each achievement to get the allotted gamerscore.
Completionist (25): Complete all Pirate's Booty side missions
Gadabout (25): Discover all named locations in Oasis and the surrounding Pirate's Booty zones
Treasure Hunter (75): Complete the mission "X Marks the Spot"

Borderlands 2: Tiny Tina's Assault on Dragon Keep Achievements
Achievement: Description: Value
Dang Girl You Ace At This Game (30): Win the most challenging round in Murderlin's Temple
Girl's Gotta Eat (15): Feed thy noble queen 3 times during one visit to her quarters
Hmmmmm (15): Wield the Mysterious Amulet
I Totes Planned That Boss (10): Slay Mister Boney Pants Guy
It's Like That One Video (10): Show thine worst enemy, the abomination known as "The Darkness," who is the nerdiest of them all
Keep Rollin' Rollin' Rollin' (15): Demonstrate your skill, or lack thereof, at rolling the magical treasure orb of many sides
Make it Raaaaaid (50): Vanquish the Ancient Dragons of Destruction
Shorty, You So Best (60): Complete thy quest by rescuing yonder queen
They Was All "Hey That's Mine" (20): Unsheath 5 swords from Immortal Skeletaurs without leaving the area
Yaaaaaay (25): Introduce thyself to the White Knight

Minecraft enemies and outfit
In the Caustic Caverns underneath Sanctuary, near the furthest edge, there is a shallow incline. If you go along the left-most cliff beside the incline, you will find Minecraft blocks. Breaking them will lead into an area with creepers from the Minecraft games, a golden ore block with a normal outfit, and a special outfit for defeating the creepers.

Carry one more item in your backpack
You must have 2 class mods for this to work. Once your backpack is full, switch your currently equipped class mod for another one in your backpack (you can then re-equip your original class mod after you've done this). Then exit the backpack menu and you can now pick up one more item and have a backpack with 28 items in it instead of maximum 27.